Numerical study of clustered reactive media cylinders for remediating contaminated groundwater

Capture zone equations for a multi-well system in strip-shaped confined and unconfined aquifers with and without regional flow are presented. The aquifer is limited by two parallel boundaries that are either no flow (barrier) or inflow (variable head) so that aquifers with four possible boundary configurations are formed. The wellfield includes any number of extraction or injection wells or a combination of both types. The flow field in the strip-shaped aquifer was converted to its equivalent extensive aquifer using conformal mapping and image well methods. To delineate the capture envelope, the potential, streamline and stagnation point equations were derived using velocity potential theory. The solution permits rapid determination of the effect of number, position and extraction/injection rate of wells, boundary type and direction, and rate of regional flow on the size, shape and pattern of well capture zones. The derived equations are readily extended to water quality and quantity management simulations, as shown by embedding the equations within two optimization schemes, viz., Particle Swarm Optimization (PSO) and Genetic Algorithm (GA), to automatically determine the most efficient wellfield designs for pump-and-treat remediation, contaminant plume containment and pumping policy projects.

Alternative configurations of non-pumped wells filled with reactive media were evaluated for removing hypothetical contaminant plumes. All wells were screened across the saturated zone of a simulated unconfined aquifer. Three heterogeneous hydraulic conductivity distributions (cases) were considered. A mass transport model accounting for advection and hydrodynamic dispersion produced an initial contaminant plume for each case. Two reactive well configurations were evaluated for each case. In one configuration, evenly spaced wells occupied a linear transect perpendicular to regional groundwater flow, located downgradient of the contaminant plume. A second configuration involved the same number of wells, but along evenly spaced, nonlinear flow lines originating from the downgradient boundary of the contaminant plume. Mass transport modeling simulated contaminant plumes moving through the aquifer and wells. Results suggest that nonlinear configurations, which take into account local flow variations near the downgradient boundary of a contaminant plume, more efficiently reduce contaminant concentrations and better control offsite migration.

The Chromic Acid Pit site is an inactive waste disposal site that is regulated by the Resource Conservation and Recovery Act of 1976. The 2.2-cubic-yard cement-lined pit was operated from 1980 to 1983 by a contractor to the U.S. Army Air Defense Artillery Center and Fort Bliss. The pit, located on the Fort Bliss military reservation, in El Paso, Texas, was used for disposal and evaporation of chromic acid waste generated from chrome plating operations. The site was certified closed in 1989 and the Texas Natural Resources Conservation Commission issued Permit Number HW-50296 (U.S. Environmental Protection Agency Permit Number TX4213720101), which approved and implemented post-closure care for the Chromic Acid Pit site. In accordance with an approved post-closure plan, the U.S. Geological Survey is cooperating with the U.S. Army in evaluating hydrogeologic conditions and ground-water quality at the site. One upgradient and two downgradient ground-water monitoring wells were installed adjacent to the chromic acid pit by a private contractor. Quarterly ground-water sampling of these wells by the U.S. Geological Survey began in December 1993. The Chromic Acid Pit site is situated in the Hueco Bolson intermontane valley. The Hueco Bolson is a primary source of ground water in the El Paso area. City of El Paso and U.S. Army water-supply wells are located on all sides of the study area and are completed 600 to more than 1,200 feet below land surface. The ground-water level in the area of the Chromic Acid Pit site has declined about 25 feet from 1982 to 1993. Depth to water at the Chromic Acid Pit site in September 1994 was about 284 feet below land surface; ground-water flow is to the southeast. Ground-water samples collected from monitoring wells at the Chromic Acid Pit site contained dissolved-solids concentrations of 442 to 564 milligrams per liter. Nitrate as nitrogen concentrations ranged from 2.1 to 2.7 milligrams per liter; nitrite plus nitrate as nitrogen concentrations ranged from 2.3 to 3.0 milligrams per liter. Nitrate concentrations are abnormally high in the Old Mesa well field located about 5,000 feet southwest of the Chromic Acid Pit site. Volatile and semivolatile organic compounds in water samples were analyzed for the first sampling round; no confirmed volatile or semivolatile organic compounds were detected above the laboratory reporting limits. Total chromium concentrations ranged from 0.0099 to 0.092 milligram per liter; dissolved chromium concentrations ranged from 0.0068 to 0.0094 milligram per liter. Overall, water-quality characteristics in water from the chromic acid pit ground-water monitoring wells are similar to those in the surrounding area. Detected chemical concentrations in water from the chromic acid pit monitoring wells during the four sampling periods were below U.S. Environmental Protection Agency-established maximum contaminant levels for public drinking water supplies. Statistical analyses were performed on 39 of the chemical constituents analyzed for in ground water from the chromic acid pit monitoring wells. Concentrations of chloride and fluoride were significantly less in water from the downgradient wells than in water from the upgradient well, whereas concentrations of nitrate as nitrogen, nitrite plus nitrate as nitrogen, and dissolved solids were significantly greater in water from the downgradient wells than in water from the upgradient well. Concentrations of nitrate as nitrogen were significantly different in water from the two downgradient wells. Differences detected through statistical analysis of chemical constituents of water in the chromic acid pit monitoring wells did not appear to indicate a release of hazardous chemicals from the chromic acid pit. There was no indication of ground-water contamination in either downgradient well.

A groundwater flow and mass transport model tested the capability of shallow excavations filled with coarse, reactive media to remediate a hypothetical unconfined aquifer with a maximum saturated thickness of 5 m. Modeled as contaminant sinks, the rectangular excavations were 10 m downgradient of an initial contaminant plume originating from a source at the top of the aquifer. The initial plume was approximately 259 m long, 23 m wide, and 5 m thick, with a downgradient tip located approximately 100 m upgradient of the site boundary. The smallest trench capable of preventing offsite migration was 11 m long (measured perpendicular to groundwater flow), 4 m wide (measured parallel to groundwater flow), and 3 m deep. Results of this study suggest that shallow trenches filled with coarse filter media that partially penetrate unconfined aquifers may be a viable alternative for remediating contaminated groundwater at some sites.

Modeling of strategies for performance monitoring of groundwater contamination at sites underlain by fractured bedrock

Spatially Averaged, Flow‐Weighted Concentrations – A More Relevant Regulatory Metric for Groundwater Cleanup

Optimizing short-term plume containment: comparison of well arrangements : Suguino, H H; Peralta, R C Proc Symposium on Geology and Hydrology of Hazardous-Waste, Mining-Waste, Waste-Water and Repositories Sites in Utah, Salt Lake City, 1989P43–50. Publ Salt Lake City: Symposium and Field Conference, 1989 (Utah Geological Association Publication 17)

Reliability assessment of groundwater monitoring networks at landfill sites

The groundwater flow and solute transport models were established by Visual Modflow, which was used to forecast the transport process of Cr6+ in groundwater and simulate the effects of three control measures of contaminants transport after percolation solution leakage happened in the impermeable layer of the landfill. The results showed that the contamination plume of Cr6+ would reach the pool’s boundary in 10 years, and the distance of contamination transport was 1,450 m. However, the contamination plume will not be obviously expanded between 10 and 20 years. While the ground was covered by hardened concrete, the contamination plume would not reach the pool’s boundary in 20 years. When the leakage-proof barrier was set in the bottom of an unconfined aquifer, the concentration of Cr6+ was higher than that of the leakage-proof barrier unset, but the result was opposite when setting the leakage-proof barrier in the bottom of confined aquifer. The range of the contamination plume was effectively controlled by setting drainage ditches in which water discharge was 2,298.05 m3 d−1, which produced monitoring wells which are not contaminated in 20 years. In sum, combining ground hardening with drainage ditches could produce the best effect in controlling contaminants diffusion, and meanwhile, the drainage ditches daily discharge was reduced to 1,710.19 m3 d−1.

A groundwater plume containing high concentrations of pharmaceutical compounds, mainly sulfonamides, barbiturates, and ethyl urethane, in addition to chlorinated ethenes and benzene was investigated. The contamination originating from a former pharmaceutical industry discharges into a multilayered aquifer system and a downgradient stream. In this study, geological and hydrogeological data were integrated into a numerical flow model to examine identified trends using statistical approaches, including principal component analysis and hierarchal cluster analysis. A joint interpretation of the groundwater flow paths and contaminant concentrations in the different compartments (i.e., groundwater and hyporheic zone) provided insight on the transport processes of the different contaminant plumes to the stream. The analysis of historical groundwater concentrations of pharmaceutical compounds at the site suggested these compounds are slowly degrading. The pharmaceutical compounds migrate in both a deep semiconfined aquifer, as well as in the shallow unconfined aquifer, and enter the stream along a 2-km stretch. This contrasted with the chlorinated ethenes, which mainly discharge to the stream as a focused plume from the unconfined aquifer. The integrated approach developed here, combining groundwater flow modeling and statistical analyses of the contaminant concentration data collected in groundwater and the hyporheic zone, lead to an improved understanding of the observed distribution of contaminants in the unconfined and semiconfined aquifers, and thus to their discharge to the stream. This approach is particularly relevant for large and long-lasting contaminant sources and plumes, such as abandoned landfills and industrial production sites, where field investigations may be very expensive.

The publicly available analytical contaminant transport analysis system (ACTS) was used to examine the fate and transport of organochlorine pesticides in shallow groundwater at the Oatland Island Education Center, Oatland Island, Georgia. Specific objectives of the analysis included: (1) estimating the probability of impacting coastal wetlands located 800 feet (244 meters) downgradient of the pesticide source area, and (2) developing reference tools (probability curves) for evaluating future groundwater monitoring results at key site monitoring wells. The groundwater contaminant plume consists of four different isomers of benzene hexachloride (BHC). Total BHC was selected as the reference contaminant for the contaminant, fate and transport model simulations. Probabilistic model results for each specific site location and time period were generated by conducting two-stage Monte Carlo simulations for eight model parameters using 10,000 realizations for each probability distribution. Model results are considered representative of all four BHC isomers detected at the site. According to simulation results at the wetlands boundary, the probability of total BHC concentration in groundwater exceeding the analytical detection limit of 0.04 micrograms per liter (μg/L) is less than 2%. The detection limit of 0.04 μg/L is a default compliance standard for BHCs in groundwater, in accordance with applicable Georgia Environmental Protection Division rules. Model simulation results were used to develop a series of probability curves for four different site locations — three downgradient monitoring wells and the downgradient wetlands boundary — along the centerline of the contaminant plume. These probability curves are useful in evaluating future groundwater monitoring results and guiding environmental and regulatory decision-making for the site.

Numerical modelling and performance evaluation of multi-permeable reactive barrier system for aquifer remediation susceptible to chloride contamination

Selected hydrologic data for Salt Lake Valley, Utah, 1990-92, with emphasis on data from the shallow unconfined aquifer and confining layers

　　Certain limitations arise when utilizing the Monitoring Efficiency Model (MEMO) and Monitoring and Remediation Optimization System (MAROS) to evaluate monitoring well placement at contaminated sites. MEMO is restricted to one-dimensional groundwater flow analysis, while MAROS can only handle two-dimensional spatial distribution of contaminants. These constraints hinder the ability to account for variability in the three-dimensional spatial distribution of contaminants, leading to suboptimal monitoring well configurations. In particular, factors such as geological heterogeneity and contaminant characteristics (e.g., biodegradation, chemical degradation, and physical adsorption) may lead to contaminant omissions or inappropriate monitoring well density distribution, ultimately limiting the efficiency and accuracy of monitoring well placement.　　To address these challenges, this study proposes an optimized approach for monitoring well placement at three-dimensional groundwater contamination sites. The method integrates Bayesian Model Averaging (BMA) and Bayesian Maximum Entropy (BME) to delineate contaminant plumes more accurately and provide optimal recommendations for monitoring well placement. BMA, utilizing Markov Chain Monte Carlo (MCMC) simulations and Bayesian inference, calculates the posterior distribution of multiple potential Conceptual Site Models (CSMs) by evaluating discrepancies between observed and simulated contaminant concentrations.　　Using the weighted CSM, the relative positions between existing monitoring wells and the contaminant plume can be evaluated. During the numerical simulation process, virtual observation points are added to enhance the richness and completeness of data distribution within the contaminated area, further improving the interpolation accuracy of BME. Through this improvement, BME can integrate simulated data with existing monitoring data to precisely predict the locations of additional monitoring wells, supplement critical monitoring data, and optimize the overall monitoring well placement strategy.　　Additionally, this study incorporates monitoring well-installation costs, the value of information (VOI), and trans-information entropy (TE) into a multi-objective optimization framework. By minimizing the objective function, Pareto-optimal solutions are obtained. The Preference Ranking Organization METHod for Enrichment Evaluations (PROMETHEE) is then applied to rank these solutions, enabling decision-makers to balance monitoring efficiency with cost considerations and implement flexible and effective monitoring configurations. It also verifies the feasibility of retaining a significant portion of critical monitoring information through VOI-based quantitative analysis, even with a reduced number of monitoring wells.　　The proposed optimization method has been validated through numerical simulations, demonstrating improved model accuracy under complex site conditions. The results offer adaptable, site-specific solutions that maximize both monitoring efficiency and economic viability. Keywords: Bayesian Model Averaging, Bayesian Maximum Entropy, groundwater contaminant transport, optimization of monitoring well placement

In the field of groundwater, accurate delineation of contaminant plumes is critical for designing effective remediation strategies. Typically, this identification poses a challenge as it involves solving an inverse problem with limited concentration data available. To improve the understanding of contaminant behavior within aquifers, hydrogeophysics emerges as a powerful tool by enabling the combination of non-invasive geophysical techniques (i.e., electrical resistivity tomography—ERT) and hydrological variables. This paper investigates the potential of the Ensemble Smoother with Multiple Data Assimilation method to address the inverse problem at hand by simultaneously assimilating observed ERT data and scattered concentration values from monitoring wells. A novelty aspect is the integration of a Convolutional Neural Network (CNN) to replace and expedite the expensive geophysical forward model. The proposed approach is applied to a synthetic case study, simulating a tracer test in an unconfined aquifer. Five scenarios are compared, allowing to explore the effects of combining multiple data sources and their abundance. The outcomes highlight the efficacy of the proposed approach in estimating the spatial distribution of a concentration plume. Notably, the scenario integrating apparent resistivity with concentration values emerges as the most promising, as long as there are enough concentration data. This underlines the importance of adopting a comprehensive approach to tracer plume mapping by leveraging different types of information. Additionally, a comparison was conducted between the inverse procedure solved using the full geophysical forward model and the CNN model, showcasing comparable performance in terms of results, but with a significant acceleration in computational time.